In the gas phase process for production of polyolefins such as polyethylene, a gaseous alkene (e.g., ethylene), hydrogen, co-monomer and other raw materials are converted to solid polyolefin product. Generally, gas phase reactors include a fluidized bed reactor, a compressor, and a cooler (heat exchanger). The reaction is maintained in a two-phase fluidized bed of granular polyethylene and gaseous reactants by the fluidizing gas which is passed through a distributor plate near the bottom of the reactor vessel. Catalyst is injected into the fluidized bed. Heat of reaction is transferred to the circulating gas stream. This gas stream is compressed and cooled in the external recycle line and then is reintroduced into the bottom of the reactor where it passes through a distributor plate. Make-up feedstreams are added to maintain the desired reactant concentrations.
Operation of most reactor systems is critically dependent upon good mixing for uniform reactor conditions, heat removal, and effective catalyst performance. The process must be controllable and capable of a high production rate. In general, the higher the operating temperature, the greater the capability to achieve high production rate. Because polymerization reactions are typically exothermic, heat transfer out of the reactor is critical to avoid such things as particle agglomeration and runaway reactions. However, as the operating temperature approaches and exceeds the melting point of the polyolefin product, the particles of polyolefin become tacky and melt. For example, non-uniform fluidization of the bed can create “hot spots,” which in turn can cause the newly-formed polymer particles to become tacky due to elevated temperatures in the hot spots.
An interplay of forces may result in particles agglomerating with adjacent particles, and may lead to sheeting. In agglomeration, the particles stick together, forming agglomerated particles that affect fluid flow and may be difficult to remove from the system. In sheeting, tacky particles gather on a surface of the reactor system, such as the wall of the reactor vessel, forming a sheet of polymer particles. Progressive cycles in this process may eventually result in the growth of the sheet and its falling into the fluid bed. These sheets can interrupt fluidization, circulation of gas and withdrawal of the product from the reactor, and may require a reactor shutdown for removal.
Prior attempts at reducing sheeting include addition of antistatic agents to the catalyst or fluidized bed itself. Other approaches rely on addition of continuity additives to minimize agglomeration and sheeting. One disadvantage in using continuity additives or antistatic agents is the increased cost they add to the polymerization reaction. Another disadvantage in using continuity additives or antistatic agents is the gas phase reactor may require additional equipment to feed and monitor the level of these additives. Furthermore, the addition of material to the reactor itself tends to require complex monitoring to control the feed rate of the additive.
Therefore, there is a need for improved catalysts and methods for using the same that produce polyolefin products in gas phase fluidized bed reactors that reduce the probability of sheeting and/or agglomeration in the reactor system, and/or reduce or eliminate the need for continuity additives and/or antistatic agents.